So perhaps it's time to learn.
If You want a bunker-buster, then it doesn't need to be a nuke, but just a relatively big conventional bomb that will be dropped from high altitude and hit the ground with high speed (it would gain during a free-fall, and perhaps thanks to rocket booster in the final phase - but that's really not essential), allowing it to deeply penertate the ground. If You want a nuclear bunker-buster, then it doesn't really need to be big.
Large size nukes are mainly for area attacks (therefore high yield H-bombs are called city-killers), but they explode above the ground (for the bombs dropped on Hiroshima and Nagasaki it was several hundred feet, if I remember correctly).